In order to be suitable as a tensile member for communication cables, a number of different severe requirements must be met. The tensile member must be flexible, must be hundred per cent watertight and, although it must have a determined elastic elongation, its elongation must not exceed a predetermined value, since otherwise the metal conductors or the optical fibres could be put under a tensile strain, which is to be avoided. The tensile member is further required to withstand compressive forces and to be of low cost.
The prior art provides a die-formed steel strand as a tensile member for communication cables. A die-formed steel strand comprises a core wire and a layer of layer wires which have been plastically compressed by means of a die. This plastical compressing is done in order to obtain a high tensile modulus. The drawback, however, is that longitudinal capillarities remain between the plastically deformed layer wires and the core wire. As a consequence, watertightness is not guaranteed in the longitudinal direction of the steel strand. A prior art solution has tried to overcome the problem of watertightness by injecting bitumen during the twisting process of the individual steel wires. This solution has the drawback that if the steel strand is heated prior to an extrusion process in order to enhance the adhesion of the steel strand to the extruded synthetic material, the bitumen melts and at least part of the bitumen gets lost before extrusion, which results in uncomplete filled capillarities and in a dirty work floor. Complete watertightness is not guaranteed in this case. Non-heating of the steel strand prior to extrusion, holds the bitumen in the steel strand but decreases the adhesion of the steel strand to the extruded synthetic material. It goes without saying that loss of adhesion between the steel strand and the synthetic material is at the expense of the watertightness again.
It is an object of the present invention to avoid the drawbacks of the prior art. It is an object of the present invention to provide for a communication cable, the tensile member of which is watertight in longitudinal direction, has a limited elastic elongation and is of low cost.